The invention relates in general to partition structures for buildings and more particularly to a fire resistant ceiling or wall and method of installing the same.
Generally speaking, the area most vulnerable to a fire within a building is the ceiling directly above the fire. In the basement of a typical residential building that ceiling is usually nothing more than a series of wood floor joists over which a wood subfloor extends. When a fire develops beneath a ceiling of this construction, the flames impinge directly against the joists, setting them ablaze, and they are consumed quite rapidly, causing the floor above to collapse. Indeed, an unprotected wood joist or truss ceiling will usually fail within 10 to 12 minutes when subjected to the ASTM E119 fire test.
Ceiling constructions that include gypsum wallboard fare somewhat better, because the wallboard, being noncombustible, acts as a shield and prevents the flames from impinging directly against the wood joists--at least at the outset. Even so, a ceiling construction that includes wallboard hung in the conventional manner from joists that are spaced more than 16 inches apart, as is common in construction today, still will not pass a so-called one-hour fire test. In other words, a ceiling construction of that character, when loaded and subjected to a fire under the controlled conditions prescribed by ASTM E119, will collapse within less than one hour. The failure usually commences at the joints between the sheets of wallboard, for the intense heat destroys the bond between the joint cement and the wallboard, causing the joint cement and the tape, which is embedded in it, to fall. This opens the joint and allows the hot gases and the flames to enter the plenum normally isolated by the wallboard. The heat of the fire further drives the water of hydration from the gypsum, and as a consequence the gypsum wallboard shrinks. This opens the joints still further and thus renders the plenum region more vulnerable to the hot gases and flames. Just as significant, it causes the wallboard to draw away from the nails or screws which secure it, so that the wallboard loses its structural integrity in the regions where it is supported. As a consequence, the wallboard falls and completely exposes the wood joists to the flames.
It is not uncommon for building codes or insurance policies to require somewhat greater resistance to fire in the case of multifamily residential buildings or commercial and industrial buildings. Typically, they must pass a one-hour or longer fire test.
Several procedures exist for increasing the fire resistance of a wood joist or wood truss ceiling construction clad with wallboard. Perhaps the simplest is to merely use a double layer of wallboard, with the sheets of the lower layer being placed with their longitudinal axes oriented at 90.degree. with respect to the longitudinal axes for the sheets of the upper layer. This almost doubles the cost of hanging the wallboard for the ceiling. Another is to erect a grid against the lower surfaces of the joists or trusses and to secure the wallboard sheets to the grid instead of directly to the joists or trusses. To be effective the grid must exist behind each joint in the wallboard ceiling, so that the joint remains blocked should the tape and joint cement fall away from it. Thus, the construction of the grid requires a considerable amount of measuring, and is likewise costly. Still another procedure is to install wood blocks behind the joints to keep the joints sealed after the loosening of the joint cement and tape, but this requires a considerable amount of additional carpentry and is also costly.